1. Field of the Invention
The present invention relates to technology for inspecting an optical disk having a wobble track which has been phase-modulated, and to technology for reproducing data recorded on an optical disk.
2. Description of Related Art
Conventionally, a technique for inspecting an optical disk having wobble tracks which are wobbled in accordance with a frequency which is phase-modulated, such as DVD+R and DVD+RW, to determine whether or not the wobble tracks are formed correctly, has been proposed.
Japanese Patent Laid-Open Publication No. 2004-280878 discloses that a light beam is irradiated onto a wobble track and a wobble signal obtained from light reflected from the wobble track is doubled, whereby the quality of the wobble track is evaluated based on the frequency characteristics of the doubled signal.
FIG. 7 is a block diagram showing the structure of an evaluation device described in the above-described publication. Referring to FIG. 7, a PUH 40 irradiates a light beam onto a wobble track of an optical disk D1. A PD 50 detects light reflected from the wobble track. A radial push-pull signal which is generated from a signal detected by the PD 50, namely a wobble signal, is input to a frequency characteristics measuring section 100.
FIG. 8 is a block diagram showing the structure of the frequency characteristics measuring section 100 in FIG. 7. A low noise eliminator/amplifier 101 eliminates a direct-current component contained in the wobble signal and appropriately amplifies the wobble signal. A band pass filter 102 eliminates excessive frequency components contained in the wobble signal which is supplied thereto. Here, the excessive frequency components refer to frequency components which are sufficiently far from the carrier frequency. A multiplication circuit 103 multiplies the supplied wobble signal to generate a doubled wobble signal. A frequency characteristics measuring circuit 104 measures a doubled NBSNR (Narrow-Band Signal-to-Noise Ratio) (or CNR (Carrier Noise Ratio) to determine whether or not the evaluation result of the doubled wobble signal satisfies a predetermined evaluation target. If a difference between a peak level obtained from the frequency characteristics of the doubled wobble signal and a noise level (i.e. a doubled NBSNR or CNR) is 17 dB or greater, it is determined that the quality of the wobble track is acceptable.
As described above, it is possible to evaluate the frequency characteristics of a doubled wobble signal to thereby evaluate the quality of a wobble track. In phase-modulated wobble tracks, however, evaluation as to whether or not phases are correctly shifted by 180 degrees with respect to each other, in addition to evaluation of the NBSNR (or CNR; hereinafter collectively referred to as CNR) is also required. It is therefore difficult to evaluate a phase shift only by the measurement of CNR of a doubled wobble signal using a spectrum analyzer.